1. Field of the Invention
The present invention relates to conversion of technetium that is present in aqueous nuclear waste to a less mobile and soluble form to meet long-term disposal requirements.
2. Description of the Related Art
Technetium (Tc) is a man-made element that is generated from the fission of 235U-enriched nuclear fuel in nuclear power reactors. The most abundant technetium isotope in the radioactive wastes generated during the used nuclear fuel reprocessing activities, 99Tc, has a high fission yield (approximately 6% of total fission products) and a long half-life (213,000 years). Large amounts of 99Tc typically are stored on-site at various reprocessing locations, awaiting retrieval, processing, and permanent disposal.
Technetium has an inherent high level of reactivity in the presence of oxygen, and forms the extremely soluble and mobile pertechnetate anion. Consequently, it is difficult to immobilize into a final waste form for geologic disposal, creating a potential human health risk when present in the environment. Additionally, technetium compounds are volatile in thermal processes used for processing radioactive waste, such as vitrification, resulting in high levels of 99Tc in secondary waste streams.
Among the numerous valence states of technetium, two are predominant at standard conditions: Tc(IV) and Tc(VII), with Tc(IV) compounds (such as TcO2) being far less soluble than Tc(VII) compounds (such as NaTcO4). Proposed waste forms that incorporate 99Tc into their structure attempt to take advantage of this fact by utilizing reducing agents to form the less soluble Tc(IV) and prevent re-oxidation to the more soluble species. These getters include goethite, tin apatite, and zero valent iron. Waste forms investigated for 99Tc incorporation include Fluidized Bed Steam Reforming (FBSR), cast stone, ceramacrete, geopolymer, ion exchange resins, and others. These methods have had varying success, however, in demonstrating sufficient retention capabilities for 99Tc to prevent the mobile 99Tc from being released from the waste forms and migrating to the environment. Any waste form containing 99Tc must meet a rigorous testing program to comply with waste acceptance criteria at nuclear waste disposal facilities.
What is needed is a reliable way to safely convert technetium to a less mobile and less soluble form for long-term disposal.